When asked about his use of a GPS watch, which started in 2009 and has gone through a couple of different Garmin models, Harvey said, “I like to think I have a responsible relationship with my GPS watch.”

Living in a data-driven, quantified-self world, it’s easy to understand the appeal of gadgets that log daily activities and workouts. From the fitness trackers used by casual exercisers for motivation to the GPS watches and heart-rate monitors worn by athletes to better gauge effort, the data created by devices can become addictive. Feeling the Fitbit buzz when goals are accomplished or scrolling through mile splits recorded by a GPS watch can be oddly satisfying.

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But do they help or hurt performance? Do they make it more or less likely that casual exercisers or athletes will accomplish what they want? For many, the devices are clearly a plus, but exercise specialists warn that dependence or improper use can hinder progress or possibly even lead to injury.

Broadly speaking, there are three types of activity-workout tracking devices that have become commonplace in the last decade: accelerometers (popularized as fitness trackers), heart-rate monitors, and GPS watches. Over the years, they have become smaller, more wearable, more accurate, more affordable and, as a result, more widely used. Harvey has seen rapid growth in GPS watch usage during the last five years and figures that at least half the runners he trains with on the B.A.A. Running Club wear some kind of GPS watch to track distance and pace and trace routes.

For people starting to exercise or trying to add more activity to their daily routines, step-counting accelerometers, which measure both movement and calories burned, are good options. For exercisers and athletes focused on improving fitness and performance, heart-rate monitors and GPS watches can provide valuable feedback.

Dr. Aaron Baggish, associate director of the Cardiovascular Performance Program at Massachusetts General Hospital, regularly tests new devices at his lab. He likes the combination of a heart-rate monitor and a GPS watch for athletes, one tracking the body’s internal effort and the other tracking external effort. And he is not alone. This spring, Epson, a company best known for computer printers, will start selling its Runsense GPS watches which combine GPS functions with a heart-rate monitor that doesn’t require a chest strap. Meb Keflezighi, the 2014 Boston Marathon champion, has agreed to endorse the product.

But sometimes less is more. It’s not always about a hard effort for avid amateur athletes who use GPS watches or heart-rate monitors or both as training aids.

“Where my GPS watch has really helped me is by forcing me to go easy on my easy days,” said Wayne Levy, an accomplished marathoner and owner of running tour company RunBoston. “There are some days when you feel great, and you don’t realize how fast you’re running. By monitoring my pace per mile, I sometimes tell myself I need to slow down.”

Kristin Johnson, who won gold in a Cambridge Boat Club quad at the 2012 US Rowing Masters National Championships, uses a heart-rate monitor. While other members of her club “are obsessed with data and they have great results,” Johnson tries to keep her monitor numbers in perspective.

“I use it primarily for cardiovascular stuff, when cycling and rowing,” said Johnson. “I do that because I tend to overtrain.

“[The heart-rate monitor] gives me direction. There is really good information that can help inform your choices of a workout structure. But at a certain point, I think you’re just geeking out on the data and fooling yourself if you think it’s making you faster.”

And there’s the rub. Coaches, trainers, and other experts warn that leaning too heavily on the data generated by the devices can get in the way.

For starters, while there isn’t an abundance of hard research available on the devices, a few recent studies and news reports have cast some doubt on the accuracy of some accelerometers and heart-rate monitors.

In the case of accelerometers, or fitness trackers, at least one study suggests that the accuracy of their measurements of calorie expenditure vary somewhat from model to model.

Other researchers have found that the devices work better in tracking strenuous workouts but may underestimate it for lighter-intensity activities. Exercise scientists say this could prove discouraging to those who are trying to become less sedentary and more generally active.

As for heart-rate monitors, one recent test for the CNET tech news website by Dr. Jon Zaroff, a cardiologist at Kaiser Permanente medical center in San Francisco, found that the proliferating wrist-based sensors appear less accurate than chest-strap models.

Exercise experts say that a bigger potential problem is not the equipment but what individuals do with the information they generate.

“The tools are only as useful as the people using them,” Baggish warns. If athletes interpret the data correctly and use it as part of a long-term training plan, then the devices can help performance. If athletes simply want instant feedback and see the data as an end in itself, then the need for numbers may hinder more than help.

“I know people who will convince themselves that it’s not worth doing a workout if they’ve left their watch at home or their watch is out of batteries, which is totally nonsense,” said Baggish. “Most people who start to use either GPS watches or heart-rate monitors enough start to learn what their bodies feel like at specific intensities. So, they can get just as good a workout without their watch. But there’s definitely a tech dependence that people get. They need to be reminded that these are just tools.”

“Who’s running the show here — your brain or your gizmo?” asked Derderian. “That’s my complaint. I am afraid that quantification is a substitute for thinking and feeling.”

Derderian says that athletes who pay attention to their bodies often can sense an oncoming physical problem that a device cannot. For example, maybe your legs feel a little heavy on a run or your stride doesn’t feel as smooth. That’s not something that will show up on a GPS watch or heart-rate monitor, but those symptoms may be indicators of an injury or overtraining.

Also, athletes who are really in tune with themselves can, for instance, feel how a light lunch or an extra cup of coffee may affect an afternoon run and adjust their routines to their own advantage.

In fact, the coach argues, most truly useful feedback can be gathered by feel and a good stopwatch. In his experience, athletes who take their pulse manually during a handful of workouts soon can guess their heart rates with accuracy comparable to any device.

After a recent track session where his runners tested wrist-mounted heart-rate monitors, Derderian said, “We’ve all become conditioned to like our little gizmos. What that produces is an illusion of accuracy. If we look at a device and it tells us what our heart rate is, then we think we know something special. If we use a program to map our run, it tells your pace exactly. That is another illusion of accuracy. As an athlete, you have to know your own body.”